How should I care for and store my cartridge when not in use?
Place empty cartridges in a sealed zip-lock bag with a moist towel (paper towel dampened with water). Do not cover print head with moist towel as this can cause mixing of the colors. Try not running your cartridge completely dry – this is not good for your printer or the cartridge.

Full cartridges need to be stored upright.  Inkjet cartridges use gravity for a feed system and rely on surface tension to keep them from leaking.  Laying a cartridge on it's side will cause the ink to move away from the print head and create an air pocket.  This will lead to your cartridge not getting ink or can cause the cartridge to dry out.